1. Technical Field
The present invention relates to a molded article of polyolefin-based resin expanded beads, and, more particularly, to a molded article of polyolefin-based resin expanded beads which has electrostatic dissipating properties.
2. Related Art
Polyolefin-based resins, such as polypropylene-based resins and polyethylene-based resins, are well-balanced in terms of mechanical strength and heat resistance. Molded articles produced by in-mold molding of expanded beads composed primarily of such a polyolefin-based resin not only have excellent properties inherent in the polyolefin-based resin but also are excellent in shock-absorbing properties and show good recovery from compressive strain. Such molded articles are therefore used in a wide variety of applications including packaging materials for electric and electronic components and shock-absorbing materials for automobiles.
However, because polyolefin-based resins, such as polypropylene-based resins, are materials with a high electrical resistance, molded articles of expanded beads composed primarily of a polyolefin-based resin are electrically chargeable. Thus, expanded beads-molded articles to which antistatic performance or conductive performance has been imparted are used as the packaging materials for electronic components and so on which must be kept away from static electricity.
In addition, the improvement in performance of electronic components, such as integrated circuits and hard disks in recent years has created a demand for a packaging material formed from an electrostatically dissipative material having a surface resistivity of 1×105 to 1×1010Ω to prevent the electronic components from being damaged by static electricity. However, a polyolefin-based resin expanded beads molded article having a surface resistivity of 1×105 to 1×1010Ω has not been able to be produced stably by a conventional antistatic or conductivity-imparting treatment technology.
For example, as methods for imparting antistatic performance to a polyolefin-based resin expanded beads molded article, Japanese Unexamined Patent Publication JP-A-H07-304895 discloses a method for producing a polypropylene-based resin expanded beads molded article by in-mold molding of antistatic expanded beads obtained by expanding resin particles containing a surfactant, and JP-A-2009-173021 discloses a method for producing a polyolefin-based resin expanded beads molded article by in-mold molding of expanded beads obtained by expanding resin particles covered with a resin layer containing a polymeric antistatic agent.
These methods are effective in producing an expanded beads molded article having a surface resistivity higher than 1×1010Ω. However, it is difficult to achieve a surface resistivity of not higher than 1×1010Ω with a method which uses a proper amount of an antistatic agent as described above because of the limitations of the electrical characteristics of the antistatic agent itself. Namely, when a large amount of an antistatic agent is added for the purpose of achieving a surface resistivity of below 1×1010Ω, the expandability and fusion bonding capability of the resulting expanded beads are considerably deteriorated.
As a further method for imparting electrical conductivity to a polyolefin-based resin expanded beads molded article, JP-A-H09-202837, JP-A-H10-251436 and JP-A-2000-169619 disclose a method in which an electrically conductive inorganic substance, such as an electrically conductive carbon black or metal powder, is added to the base resin of polypropylene-based resin expanded beads to create an electrically conductive network of an electrically conductive inorganic substance in the base resin. In addition, a method for producing an expanded beads molded article by in-mold molding of electrically conductive expanded beads obtained by expanding polypropylene-based resin particles covered with a resin layer containing an electrically conductive carbon black is disclosed in JP-A-2006-232939. These methods allow an electrically conductive molded article of polypropylene-based resin expanded beads having a surface resistivity of lower than 1×105Ω to be easily produced.